The present invention relates to a radio receiver (or a receiver section of a transceiver) having a temperature controlled crystal oscillator (TCXO) whose frequency may be further adjusted by an automatic frequency control (afc) circuit. A particular but not exclusive example of a radio receiver is a digital wide area pager which is operated in accordance with a time division protocol, such as the CCIR Radiopaging Code No. 1 (alternatively known as POCSAG).
A TCXO includes a tunable element, such as a varactor, to pull or adjust the output frequency so that it remains substantially constant in spite of the effects of temperature variation on the stability of the oscillator. If the radio receiver includes an afc circuit then its output is also coupled to the varactor. In order to maintain frequency stability in time division operation, the varactor control voltage must be held in a storage device, such as a capacitor, or generated as quickly as possible to ensure frequency stability. The temperature compensation voltage produced by say a thermistor is available substantially instantly but the afc loop has to start to generate an afc voltage which incurs a time delay.
FIG. 1 of the accompanying drawings shows one arrangement for simultaneously applying temperature compensation and afc voltages which have been generated in an integrated radio receiver circuit and are supplied to an off-chip TCXO 10 such as described in European Patent Specification EP-B1-0401919. An on-chip current source 12 representing the temperature compensating current is supplied to an inverting input 13 of a first op-amp 16 which is functioning as a current to current converter. An output 15 of the first op-amp 16 is connected by way of a switch 17 to a pin P4. The pin P4 is connected to one side of an off-chip capacitor C, the other side of which is coupled to an input of the TCXO 10. The output 15 of the op-amp 16 is coupled back to the inverting input 13 by an off-chip resistor 18 connected between the pin P4 and a pin P3 which is connected to the input 13. An off-chip bias setting resistor 20 for the TCXO's varactor is connected by way of a pin P5 to a non-inverting input 14 of the op-amp 16.
An on-chip afc current source 22 is connected to an inverting input 23 of a second op-amp 26 which is functioning as a current to current converter. A non-inverting input 24 of the op-amp 26 is connected on-chip to the output 15 of the op-3mp 16 to provide an offset for the afc. An output 25 of the op-amp 26 is fed back to the input 23 by way of an on-chip switch 27 and an off-chip resistor 28 which is connected to pins P1 and P2. A further resistor 30 is connected between the pin P1 and the input of the TCXO 10.
The resistor 18 serves to match the temperature compensation current, which is always present, to the afc current. The resistor 28 is provided to adjust the current to voltage rate. The combination of the resistor 30 and the capacitor C serve to hold the varactor bias when the switches 17 and 27 are open (or non-conductive) and thereby prevent the TCXO 10 from de-tuning.
Drawbacks to this known circuit when it is implemented as an integrated circuit are that it requires 5 pins, the switches 17 and 27 are sources of leakage currents, the op-amps 16 and 26 have to be compatible and when the outputs of the op-amps go high, the capacitor C is floating which makes it susceptible to noise from other sources.